Oversampling FIR filter, method for controlling the same, semiconductor integrated circuit having the same, and communication system for transmitting data filtered by the same

ABSTRACT

When changing the number of oversamples is performed, tap factors selected by selectors, which respectively correspond to holding parts in a shift register, are changed back to a predetermined number of tap factors used before the changing of the number of oversamples, in which every time input data is accepted, the changes of the tap factors are performed in sequence, starting from the selector corresponding to the holding part at the input side. This allows the individual selectors to select proper tap factors according to the input data after the changing of the number of oversamples. As a result, the continuity of the output data is maintained even before and after the number of oversamples is changed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an oversampling FIR filter foruse in a portable terminal, etc. of a mobile communication system.

[0003] 2. Description of the Related Art

[0004] FIR (Finite Impulse Response) filters have such linear phasecharacteristics, transfer functions, and stability that analog filterscannot achieve, and thus are put into a variety of uses includingcommunication apparatuses and audio apparatuses. In particular, FIRfilters are effectively applied to the filters on CDMA (Code DivisionMultiple Access) and other digital mobile communication methods. Forstabilization of filter rejection regions, FIR filters of oversamplingtype have become dominant.

[0005] Among the oversampling FIR filters of this type is, for example,the one disclosed in Japanese Unexamined Patent Application PublicationNo. Hei8-37444.

[0006] The oversampling FIR filter disclosed in the publicationincludes: a shift register having a plurality of delay elements forholding input data; a factor selector; and an adder. The factor selectorgenerates predetermined tap factors in accordance with the output valuesfrom the delay elements of the shift register and a timing signal. Theadder obtains the sum of the plurality of tap factors output from thefactor selector.

[0007] In an oversampling FIR filter configured as described above, theshift register need not be provided with as many delay elements as thenumber corresponding to the number of oversamples of the input data.This reduces the adder in scale, allowing a simpler hardwareconfiguration as compared with ordinary oversampling FIR filters.Incidentally, in the following description, an oversampling FIR filterof this type will be also referred to as an oversampling FIR filter oftap switching type.

[0008] In the CDMA method, a portable terminate diffuses its to-transmitsignals with diffusion codes and output the diffused data toward a basestation. Here, for the sake of phase adjustment of data received at thebase station, the portable terminal needs to change the number ofoversamples in its FIR filter depending on the distances between theportable terminal and the base station. The above-mentioned FIR filterof tap switching type does not have as many delay elements as the numbercorresponding to the number of oversamples of input data. The number ofoversamples signifies the number of times oversampling is performed.Accordingly, a change in the number of oversamples has caused a problemof discontinuity in output response. Thus, it has been impossible forFIR filters of tap switching type to be used for portable terminals inthe CDMA method with varying the number of oversamples. Meanwhile,oversampling FIR filters having as many delay elements as the number ofoversamples are greater in circuit scale as compared with theoversampling FIR filters of tap switching type. However, the number ofzero data to be added to input data can be changed in accordance withthe number of oversamples, thereby achieving the continuity in outputresponse.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide an oversamplingFIR filter of tap switching type in which the continuity of filteroutput responses is maintained even when the number of oversampling isperformed is changed while the filter is in operation.

[0010] Another object of the present invention is to reduce the circuitscale of an oversampling FIR filter.

[0011] According to one of the aspects of the present invention, a shiftregister having a plurality of holding parts connected in cascade in theFIR filter, accepts input data and shifts the accepted data in sequenceto be output from the individual holding parts. A plurality of selectorsformed corresponding to the respective holding parts sequentiallyselect, from a plurality of tap factors, a predetermined number of tapfactors in synchronization with a clock. The number of oversamples isthe number of tap factors to be multiplied during a cycle of input datavariation. In other words, the number of oversamples is the number oftap factors to be selected for data by each selector. Multipliersrespectively multiply the input data held in the holding parts by thetap factors selected by the selectors corresponding to the holdingparts. The multiplication results are added by an adder to be output asoutput data.

[0012] The oversampling FIR filter is capable of maintaining thecontinuity of output data by changing tap factors to be selected by theselectors in accordance with a change in the number of oversamples.Conventionally, a number of holding parts corresponding to the number ofoversamples are required to be provided in order to maintain thecontinuity of output data. In the present invention, it is not necessaryto provide as many holding parts as the corresponding number of timesoversampling is performed so that the oversampling FIR filter can bereduced in circuit scale.

[0013] According to another aspect of the present invention, a part ofthe plurality of tap factors selectable by the selectors is/are sharedby the selectors. Therefore, even in the cases where the tap factors tobe selected by the selectors are changed in accordance with a change inthe number of oversamples, it is able to secure the tap factors to bemultiplied by the input data with the whole selectors. As a result, thecontinuity of output data is maintained.

[0014] According to another aspect of the present invention, theselectors are instructed the tap factor to be selected first after theacceptance of input data, in accordance with a change in the number ofoversamples. The instruction is exercised by a tap controlling unitinside the filter, a tap controlling unit outside the filter orsoftware. The selectors have only to select tap factors according to theinstruction. This simplifies the control mechanism. This consequentlyallows high speed filtering operations. Moreover, the application of thepresent invention avoids a rise in cost due to the simplified controlmechanism.

[0015] According to another aspect of the present invention, when thenumber of oversamples is changed, the number of the tap factors to beselected by the selectors is changed back to the predetermined number oftap factors used before changing the number of oversamples. Morespecifically, every time input data is accepted, the changing of thenumber of tap factors are performed in sequence, starting from theselector corresponding to the holding part at the input side. Therefore,each of the selectors can select proper tap factors according to theinput data after the change of the number of oversamples. This resultsin maintaining the continuity of output data before and after the changeof the number of oversamples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The nature, principle, and utility of the invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

[0017]FIG. 1 is a block diagram showing a first embodiment of thepresent invention;

[0018]FIG. 2 is a block diagram showing the details of an FIR filter inFIG. 1;

[0019]FIG. 3 is a circuit diagram showing an example of a generator oftap trigger signals;

[0020]FIG. 4 is an explanatory diagram showing an operation of the FIRfilter in the first embodiment;

[0021]FIG. 5 is an explanatory diagram showing another operation of theFIR filter in the first embodiment;

[0022]FIG. 6 is a block diagram showing an oversampling FIR filteraccording to a second embodiment of the present invention;

[0023]FIG. 7 is an explanatory diagram showing an operation of the FIRfilter in the second embodiment;

[0024]FIG. 8 is an explanatory diagram showing another operation of theFIR filter in the second embodiment;

[0025]FIG. 9 is an explanatory diagram showing another operation of theFIR filter in the second embodiment;

[0026]FIG. 10 is a block diagram showing an oversampling FIR filteraccording to a third embodiment of the present invention;

[0027]FIG. 11 is an explanatory diagram showing an operation of the FIRfilter in the third embodiment;

[0028]FIG. 12 is an explanatory diagram showing another operation of theFIR filter in the third embodiment; and

[0029]FIG. 13 is an explanatory diagram showing another operation of theFIR filter in the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, the embodiments of the present invention will bedescribed with reference to the drawings.

[0031]FIG. 1 shows a first embodiment of the oversampling FIR filter,the method for controlling the oversampling FIR filter, thesemiconductor integrated circuit having the oversampling FIR filter, andthe communication system for transmitting data filtered by theoversampling FIR filter.

[0032] An oversampling FIR filter 2 is used, for example, in atransmitting circuit 6 of a portable terminal 4 in a CDMA or W-CDMA(Wideband-CDMA) communication system. The transmitting circuit 6 isformed in a single chip by integrating CMOS transistors and the like ona Si substrate by using semiconductor manufacturing technologies.Signals transmitted from the portable terminal 4 are received at a basestation 8.

[0033]FIG. 2 shows the details of the oversampling FIR filter 2.

[0034] The oversampling FIR filter 2 includes: a shift register 10having flip-flops FF0, FF1, FF2, FF3, and FF4 for holding input data,connected in series from the input side; selectors SEL0, SEL1, SEL2,SEL3, and SEL4 and multipliers MLT0, MLT1, MLT2, MLT3, and MLT4 formedcorresponding to the flip-flops FF0-FF4, respectively; an adder ADT; anda tap controlling unit 12.

[0035] The shift register 10 receives input data DIN at the flip-flopFF0 on the initial stage, and shifts the received data to the outputside sequentially in synchronization with a sampling trigger signalSTRG. The selector SEL0 receives tap factors C1, C2, C3, C4, and C5, andsequentially selects and outputs any of the tap factors according tocontrol signals from the tap controlling unit 12. The selector SEL1receives tap factors C4, C5, C6, C7, C8, and C9, and sequentiallyselects and outputs any of the tap factors according to control signalsfrom the tap controlling unit 12. The selector SEL2 receives tap factorsC8, C9, C10, C11, C12, and C13, and sequentially selects and outputs anyof the tap factors according to control signals from the tap controllingunit 12. The selector SEL3 receives tap factors C12, C13, C14, C15, C16,and “0”, and sequentially selects and outputs any of the tap factorsaccording to control signals from the tap controlling unit 12. Theselector SEL4 receives a tap factor C16 or “0”, and selects and outputseither of the tap factors according to control signals from the tapcontrolling unit 12. Here, the selectors SEL0 and SEL1 both receive thetap factors C4 and C5, for example. Both the selectors SEL1 and SEL2receive the tap factors C8 and C9. Likewise, in the present embodiment,selectors adjacent to one another share a part of tap factors. That isto say, the selectors SEL0-SEL4, have a configuration significantlydifferent from conventional ones.

[0036] The multipliers MLT0-MLT4 multiply the input data DIN output fromthe flip-flops FF0-FF4 by the tap factors output from the selectorsSEL0-SEL4, respectively, and output the results of the multiplication tothe adder ADT. The adder ADT adds the multiplication results from themultipliers MLT0-MLT4 in sequence, and outputs the resultant as outputdata DOUT.

[0037] The tap controlling unit 12 receives the sampling trigger signalSTRG, a clock signal CLK, and a tap trigger signal TTRG, and outputscontrol signals to the individual selectors SEL0-SEL4. The samplingtrigger signal STRG is a signal to shift the input data DIN in the shiftregister 10. The clock signal CLK is a signal to switch the tap factorssequentially in the selectors SEL0-SEL4. The tap trigger signal TTRG isa signal to increment or decrement by one the number of oversamples,which signifies the number of times oversampling is performed. In thisembodiment, the tap trigger signal TTRG has a frequency four timeshigher than the frequency of the clock signal CLK. That is, this FIRfilter 2 operates as an FIR filter with sixteen taps for four-timesoversampling. In other words, each time the shift register 10 shifts theinput data DIN, the selectors SEL0-SEL4 each makes four selections oftap factors.

[0038]FIG. 3 shows an example of a generator of the tap trigger signalTTRG. This generator 16 is formed, for example, in the transmittingcircuit 6 shown in FIG. 1.

[0039] This generator 16 has: a binary counter 16 a which is reset bythe sampling trigger signal STRG and counts up the clock signal CLK; andAND circuits 16 b and 16 c which generate tap trigger signals TTRG(−)and TTRG(+), respectively. The AND circuit 16 b outputs the tap triggersignal TTRG(−) while the sampling trigger signal STRG and the countervalue “3” of the counter 16 a are activated. The AND circuit 16 coutputs the tap trigger signal TTRG(+) while the sampling trigger signalSTRG and the counter value “5” of the counter 16 a are activated. Thetap trigger signals TTRG(−) and TTRG(+) are supplied as the tap triggersignal TTRG to the tap controlling unit 12 shown in FIG. 2. Then, thetap trigger signal TTRG(−) or TTRG(+) is activated when the number ofoversamples is changed from four (standard) to three or from four tofive, respectively.

[0040] Next, description will be given of the operation of theabove-described FIR filter.

[0041]FIG. 4 shows a case where the number of oversamples is changedfrom four to three while the FIR filter is in operation.

[0042] Under the control of the tap controlling unit 12, the selectorsSEL0-SEL4 shown in FIG. 2 sequentially switch the tap factors insynchronization with the clock signal CLK. Besides, under the control ofthe tap controlling unit 12, the selectors SEL0-SEL4 selectpredetermined tap factors in synchronization with the sampling triggersignal STRG. The numerals in frames indicate the number of the tapfactors selected by the selectors SEL0-SEL4.

[0043] For example, while the tap trigger signal TTRG is inactivated,the selectors SEL0-SEL4 select the tap factors C1, C5, C9, C9 C13, and“0”, respectively, in synchronization with the sampling trigger signalSTRG (FIG. 4(a)). Subsequently, the selectors SEL0-SEL4 select the tapfactors shown in FIG. 2 in sequence, starting from the left of thediagram until the sampling trigger signal STRG is activated. As aresult, the selectors SEL0-SEL3 select the continuous sixteen tapfactors C1-C16 as indicated with the dashed thick frames. The selectedtap factors C1-C16 are individually multiplied by the input data (DIN),Then, the multiplication results are added and output as the output data(DOUT).

[0044] On the other hand, in the case of decrementing the number ofoversamples by one, the sampling trigger signal STRG is made one clockshorter in the interval of activation. Then, the tap trigger signalTTRG(−) is activated in synchronization with the sampling trigger signal(FIG. 4(b)). Here, the selectors SEL0-SEL4 select the tap factors C1,C4, C8, C12, and C16, respectively (FIG. 4(c)). That is, in accordancewith the change in the number of oversamples, the tap controlling unit12 shown in FIG. 2 instructs the selectors SEL0-SEL4 tap factors to beselected first after the acceptance of the input data DIN. The tapfactors C4, C8, and C12 selected by the selectors SEL1, SEL2, and SEL3precede the tap factors C5, C9, and C13, respectively, when the numberof oversamples is four (standard).

[0045] Consequently, before and after the activation of the tap triggersignal TTRG(−), the selector SEL0 switches its tap factor to C1, C2, C3,C1 , C2, . . . in sequence. The selector SEL1 switches its tap factor toC5, C6, C7, C4, C5, . . . in sequence. The selector SEL2 switches itstap factor to C9, C10, C11, C8, C9, . . . in sequence. The selector SEL3switches its tap factor to C13, C14, C15, C12, C13, . . . in sequence.The selector SEL4 switches its tap factor to “0”, “0”, “0”, C16, “0”, .. . in sequence. Since the selector SEL3 does not select the tap factorC16 but C12, the number of oversamples on the corresponding input databecomes three.

[0046] On the next activation of the sampling trigger signal STRG, theselectors SEL0 and SEL1 select their standard tap factors C1 and C5,respectively (FIG. 4(d)). The selectors SEL2, SEL3, and SEL4 select thetap factors C8, C12, and C16, respectively, which are the same as theones previously selected. On the next activation of the sampling triggersignal STRG, the selectors SEL0-SEL2 select their standard tap factorsC1, C5, and C9, respectively (FIG. 4(e)). The selectors SEL3 and SEL4select the tap factors C12 and C16, respectively, which are the same asthe ones previously selected. On the next activation of the samplingtrigger signal STRG, the selectors SEL0-SEL3 select their standard tapfactors C1, C5, C9, and C13, respectively (FIG. 4(f)). The selector SEL4selects the tap factor C16 which is the same as the one previouslyselected. Then, on the next activation of the sampling trigger signalSTRG, the selectors SEL0-SEL4 select their standard tap factors C1, C5,C9, C13, and “0”, respectively.

[0047] In this way, each time the sampling trigger signal STRG isactivated, the tap factors are sequentially changed back to the standardtap factors, which starts from the selector corresponding to theinput-side flip-flop shown in FIG. 2, to decrement the number ofoversamples. Then, as shown by the thick frames in full lines, the inputdata (DIN) is multiplied by the tap factors C1-C16 in sequence andoutput as the output data (DOUT). That is, the continuity in outputresponse is maintained.

[0048]FIG. 5 shows a case where the number of oversamples is changedfrom four to five while the FIR filter is in operation. Incidentally, inthe cases of incrementing the number of oversamples, the selector SEL4selects “0”, at all times.

[0049] When the number of oversamples is four, the selectors SEL0-SEL4each perform the same operations as in FIG. 4; therefore, descriptionthereof will be omitted.

[0050] In case of incrementing the number of oversamples by one, thesampling trigger signal STRG is made one clock longer in the interval ofactivation. Then, the tap trigger signal TTRG(+) is activated insynchronization with the sampling trigger signal (FIG. 5(a)).Subsequently, the selectors SEL0-SEL3 select the tap factors C1, C6,C10, and C14, respectively (FIG. 5(b)). The tap factors C6, C10, and C14selected by the selectors SEL1-SEL3 succeed the tap factors C5, C9, andC13, respectively, when the number of oversamples is four (standard).

[0051] The selector SEL3 selecting the tap factor “0” once, makes thenumber of oversamples on the corresponding input data five.

[0052] Before and after the activation of the tap trigger signalTTRG(+), the selector SEL0 switches its tap factor to C1, C2, C3, C4,C5, C1, C2, . . . in sequence. The selector SEL1 switches its tap factorto C5, C6, C7, C8, C9, C6, C7, . . . in sequence. The selector SEL2switches its tap factor to C9, C10, C11, C12, C13, C10, C11, . . . insequence. The selector SEL3 switches its tap factor to C13, C14, C15,C16, “0”, C14, C15, . . . in sequence. Since the selector SEL3 selectsthe tap factor “0”, the number of oversamples on the corresponding inputdata becomes five.

[0053] On the next activation of the sampling trigger signal STRG, theselectors SEL0 and SEL1 select their standard tap factors C1 and C5,respectively (FIG. 5(c)). The selectors SEL2 and SEL3 select the tapfactors C10 and C14, respectively, which are the same as the onespreviously selected. On the next activation of the sampling triggersignal STRG, the selectors SEL0-SEL2 select their standard tap factorsC1, C5, and C9, respectively (FIG. 5(d)). The selector SEL3 selects thetap factor C14 which is the same as the one previously selected. On thenext activation of the sampling trigger signal STRG, the selectorsSEL0-SEL3 select their standard tap factors C1, C5, C9, and C13,respectively (FIG. 5(e)).

[0054] In this way, each time the sampling trigger signal STRG isactivated, the selectors the tap factors are changed back to thestandard tap factors, which starts from the selector corresponding tothe input-side flip-flop shown in FIG. 2, to increment the number ofoversamples. Then, as shown by the thick frames in full lines, the inputdata (DIN) is multiplied by the tap factors C1-C16 in sequence andoutput as the output data (DOUT). That is, the continuity in outputresponse is maintained.

[0055] Note that in this embodiment the number of oversamples can bechanged only once during a period over which the filter continues torespond to a single piece of input data DIN. That is, in FIG. 4, thenumber of oversamples can be decreased or increased only once during thesixteen oversampling periods (the periods over the thick frames in fulllines).

[0056] As described above, in this embodiment, the tap factors to beselected by the selectors SEL0-SEL4 are changed in accordance with achange in the number of oversamples. This ensures the continuity of theoutput data DOUT. Since the holding parts need not be provided as manyas the number corresponding to the number of oversamples, theoversampling FIR filter can be reduced in circuit scale.

[0057] A part of the tap factors selectable by the selectors SEL0-SEL4are shared. Therefore, the tap factors to be selected for input data DINcan be secured by the whole selectors, whereby the continuity of theoutput data DOUT can be maintained.

[0058] In accordance with the number of oversamples, the tap controllingunit 12 instructs the selectors SEL0-SEL4 tap factors to be selectedfirst after the acceptance of input data DIN. The selectors SEL0-SEL4have only to select tap factors according to the instructions, andtherefore the control mechanism can be simplified. This consequentlyallows high speed filtering operations. Moreover, the application of thepresent invention can prevent a rise in cost due to the simplifiedcontrol mechanism.

[0059] In response to the shift operations of the shift register 10after the instructions, the tap controlling unit 12 controls overchanging the predetermined tap factors selected by the selectorsSEL0-SEL4 back to the predetermined tap factors before the changing ofthe number of oversamples, in which the changes of the tap factors areperformed in sequence, starting from the selectors SEL0 to SEL4corresponding to the input-side hold parts FF0-FF4. Therefore, thecontinuity of the output data DOUT can be maintained before and afterthe change of the number of oversamples.

[0060]FIG. 6 shows an oversampling FIR filter 18 according to a secondembodiment of the present invention. The same circuits and signals asthose described in the first embodiment will be designated by identicalreference numbers, and detailed description thereof will be omitted.

[0061] As in the first embodiment, the oversampling FIR filter 18 isused, for example, in a transmitting circuit (semiconductor integratedcircuit) of a portable terminal in a CDMA or W-CDMA (Wideband-CDMA)communication system. In this embodiment, a tap control unit 20 isformed instead of the tap control unit 12 in the first embodiment. Theother configuration is identical to that of the first embodiment.

[0062] The number of tap factors selectable by the selectors SEL0-SEL4is obtained from the following expressions (1) and (2). The expressions(1) and (2) signify the required number of tap factors prior andsubsequent to four standard tap factors (for example, the tap factorsC5-C8 in SEL1), respectively.

OVR×(SL+1)−{(OVR−1)×SL+OVR}  (1)

OVR×(SL30 1)−(OVR−1)×(SL+1)  (2)

[0063] (OVR: the number of oversamples,

[0064] SL: the number of selectors SEL)

[0065] Specifically, the selector SEL0 can select tap factors C1, C2,C3, C4, and C5 where no tap factor is added prior to the standard tapfactors C1, C2, C3, and C4 and one added subsequent to the same. Theselector SEL1 can select C4, C5, C6, C7, C8, C9, and C10 where one tapfactor is added prior to the standard tap factors C5, C6, C7, and C8 andtwo added subsequent to the same. The selector SEL2 can select C7, C8,C9, C10, C11, C12, C13, C14, and C15 where two tap factors are added toprior to the standard tap factors C9, C10, C11, and C12 and three addedsubsequent to the same. The selector SEL3 can select C10, C11, C12, C13,C14, C15, C16, “0”, “0”, “0”, and “0” where three tap factors are addedprior to the standard tap factors C13, C14, C15, and C16 and four addedsubsequent to the same. The selector SEL4 can select tap factors “0”, .. . where four tap factors are added prior to the standard tap factors“0”, “0”, “0”, “0”, and five added subsequent to the same. As describedabove, all the tap factors subsequent to C16 are “0”.

[0066] The tap controlling unit 20, having flip-flops (not shown)corresponding to the flip-flops FF0-FF4 in the shift register 10, holdsreceived tap trigger signals TTRG(−) and TTRG(+) along with the timesthereof. According to the information held in the flip-flops, the tapcontrolling unit 20 controls the number of tap factors to be selected bythe selectors SEL0-SEL4.

[0067] Note that in this embodiment the number of oversamples can bedecremented or incremented by one a plurality of times during a periodwhere the filter continues to respond to a single piece of input dataDIN.

[0068] Here, the number of selectable tap factors obtained by theabove-mentioned expressions (1) and (2) are the numbers required for themaximum changes in the continuity limit of the filter responses.

[0069]FIG. 7 shows a case where the number of oversamples issuccessively decremented twice while the FIR filter is in operation.That is, the tap trigger signal TTRG(−) is successively activated twicein synchronization with the sampling trigger signal STRG. The operationaccompanied by the first activation of the tap trigger signal TTRG(−)(FIG. 7(a)) is identical to that of FIG. 4 described above, andtherefore description thereof will be omitted.

[0070] On the second activation of the tap trigger signal TTRG(−) (FIG.7(b)), the selectors SEL0-SEL4 select tap factors different from thoseselected upon the first activation of the tap trigger signal TTRG(−).

[0071] Initially, the selectors SEL0-SEL4 select the tap factors C1, C4,C7, C11, and C15, respectively (FIG. 7(c)). The tap factor C4 selectedby the selector SEL1 precedes the tap factor C5 when the number ofoversamples is four (standard). The tap factor C7 and C11 selected bythe selectors SEL2 and SEL3 are second previous to the tap factors C9and C13 when the number of oversamples is a standard.

[0072] Consequently, before and after the activation of the tap triggersignal TTRG(−), the selector SEL0 switches its tap factor to C1, C2, C3,C1, C2, . . . in sequence. The selector SEL1 switches its tap factor toC4, C5, C6, C4, C5, . . . in sequence. The selector SEL2 switches itstap factor to C8, C9, C10, C7, C8, . . . in sequence. The selector SEL3switches its tap factor to C12, C13, C14, C11, C12, . . . in sequence.The selector SEL4 switches its tap factor to C16, “0”, “0”, C15, C16, .. . in sequence. Since the selector SEL3 does not select the tap factorC15 but C11, the number of oversamples on the corresponding input databecomes two in the selector SEL4.

[0073] On the next activation of the sampling trigger signal STRG, theselectors SEL0 and SEL1 select their standard tap factors C1 and C5,respectively. The selector SEL2 selects the tap factor C8 subsequent tothe tap factor C7 (FIG. 7(d)). The selectors SEL3 and SEL4 select thetap factors C11 and C15, respectively, which are the same as the onespreviously selected. On the next activation of the sampling triggersignal STRG, the selectors SEL0-SEL2 select their standard tap factorsC1, C5, and C9, respectively (FIG. 7(e)). The selector SEL3 selects thetap factor C12 subsequent to the tap factor C11. The selector SEL4selects the tap factor C15 which is the same as the one previouslyselected. On the next activation of the sampling trigger signal STRG,the selectors SEL0-SEL3 select their standard tap factors C1, C5, C9,and C13, respectively (FIG. 7(f)). The selector SEL4 selects the tapfactor C16 subsequent to the tap factor C15. Then, on the nextactivation of the sampling trigger signal STRG, the selectors SEL0-SEL4select their standard tap factors C1, C5, C9, C13, and “0”, respectively(FIG. 7(g)).

[0074] As a result, the continuity in output response is maintained evenwhen the number of oversamples is successively decremented a pluralityof times.

[0075]FIG. 8 shows a case where the number of oversamples is incrementedtwice in succession while the FIR filter is in operation. That is, thetap trigger signal TTRG(+) is successively activated twice insynchronization with the sampling trigger signal STRG. The operationthat follows the first activation of the tap trigger signal TTRG(+)(FIG. 8(a)) is identical to that of FIG. 5 described above, andtherefore description thereof will be omitted. Besides, when the taptrigger signal TTRG(+) is activated, the selector SEL4 is supplied with“0” at all times. Accordingly, description of the selector SEL4 will beomitted.

[0076] On the second activation of the tap trigger signal TTRG(+) (FIG.8(b)), the selectors SEL0, SEL1, SEL2, and SEL3 select tap factorsdifferent from those selected upon the first activation of the taptrigger signal TTRG(+). These are controlled by the tap controlling unit26.

[0077] Initially, the selectors SEL0-SEL3 select the tap factors C1, C6,C11, and C15, respectively (FIG. 8(c)). The tap factor C6 selected bythe selector SEL1 and the tap factors C11 and C15 respectively selectedby the SEL2 and SEL3 succeed the tap factors C5 and secondly succeed C9and C13, when the number of oversamples is four (standard).

[0078] Before and after the activation of the tap trigger signalTTRG(+), the selector SEL0 switches its tap factor to C2, C3, C4, C5,C1, C2, . . . in sequence. The selector SEL1 switches its tap factor toC7, C8, C9, C10, C6, C7, . . . in sequence. The selector SEL2 switchesits tap factor to C11, C12, C13, C14, C11, C12, . . . in sequence. Theselector SEL3 switches its tap factor to C15, C16, “0”, “0”, C15, C16, .. . in sequence. The selector SEL3 selects the tap factor “0”, twice toprovide two data sections in which the number of oversamples on thecorresponding input data is “3”.

[0079] On the next activation of the sampling trigger signal STRG, theselectors SEL0 and SEL1 select their standard tap factors C1 and CS,respectively (FIG. 8(d)). The selector SEL2 selects the tap factor C10prior to the one previously selected. The selector SEL3 selects the tapfactor C15 which is the same as the one previously selected. On the nextactivation of the sampling trigger signal STRG, the selectors SEL0-SEL2select their standard tap factors C1, C5, and C9, respectively (FIG.8(e)). The selector SEL3 selects the tap factor C14 prior to the onepreviously selected. On the next activation of the sampling triggersignal STRG, the selectors SEL0-SEL3 select their standard tap factorsC1, C5, C9, and C13, respectively (FIG. 8(f)).

[0080] As a result, the continuity in output response is maintained evenif the number of oversamples is successively incremented a plurality oftimes.

[0081]FIG. 9 shows a case where the number of oversamples is decrementedonce from the standard and immediately incremented once from thestandard. That is, the tap trigger signal TTRG(−) is activated insynchronization with the sampling trigger signal STRG, and then TTRG(+)is activated. The operation that follows the first activation of the taptrigger signal TTRG(−) (FIG. 9(a)) is identical to that of FIG. 5described above, and therefore description thereof will be omitted.

[0082] When the tap trigger signal TTRG(+) is activated, the selectorsSEL0-SEL3 select their standard tap factors C1, C6, C9, and C13,respectively (FIG. 9(c)). The tap factor is selected by the selectorSEL0 when the number of oversamples is a standard. The tap factor C6selected by the selector SEL1 precedes the tap factor CS which isselected when the number of oversamples is a standard.

[0083] Before and after the activation of the tap trigger signalTTRG(+), the selector SEL0 switches its tap factor to C2, C3, C4, C5,C1, C2, . . . in sequence. The selector SEL1 switches its tap factor toC5, C6, C7, C8, C6, C7, . . . in sequence. The selector SEL2 switchesits tap factor to C9, C10, C1, C12, C9, C10, . . . in sequence. Theselector SEL3 switches its tap factor to C13, C14, C15, C16, C13, C14, .. . in sequence.

[0084] On the next activation of the sampling trigger signal STRG, theselectors SEL0, SEL1, and SEL3 select their standard tap factors C1, C5,and C13, respectively (FIG. 9(d)). The selector SEL2 selects the tapfactor C10 subsequent to firstly selected the tap factor C9. On the nextactivation of the sampling trigger signal STRG, the selectors SEL0-SEL2select their standard tap factors C1, C5, and C9, respectively (FIG.9(e)). The selector SEL3 selects the tap factor C14 subsequent to theone previously selected. On the next activation of the sampling triggersignal STRG, the selectors SEL0-SEL3 select their standard tap factorsC1, C5, C9, and C13, respectively (FIG. 9(f)).

[0085] As a result, the continuity in output response is maintained evenwhen the number of oversamples is decremented and incrementedsuccessively.

[0086] This embodiment can offer the same effects as those obtained fromthe first embodiment described above. Besides, in this embodiment, thedecrement/increment in the number of oversamples can be switchedsuccessively during a period where the filter continues to respond to asingle piece of input data DIN.

[0087]FIG. 10 shows an oversampling FIR filter 22 according to a thirdembodiment of the present invention. The same circuits and signals asthose described in the first embodiment will be designated by identicalreference numbers, and detailed description thereof will be omitted.

[0088] As in the first embodiment, the oversampling FIR filter 22 isused, for example, in a transmitting circuit (semiconductor integratedcircuit) of a portable terminal in a CDMA or W-CDMA (Wideband-CDMA)communication system.

[0089] The oversampling FIR filter 22 includes: a shift register 32having sixteen flip-flops FF0-FF15 for holding input data, connected inseries from the input side; selectors SEL0-SEL1 and multipliersMLT0-MLTl5 formed corresponding to the flip-flops FF0-FF15,respectively; an adder ADT; and a tap controlling unit 24. Themultipliers MLT0-MLT15 multiply the input data DIN output from theflip-flops FF0-FF15 by the tap factors output from the selectorsSEL0-SEL15, respectively, and output the results of the multiplicationto the adder ADT.

[0090] The tap controlling unit 24, having flip-flops (not shown)corresponding to the flip-flops FF0-FF15, holds a received tap triggersignal TTRG and the time thereof. Based on the information held in theflip-flops, the tap controlling unit 24 controls the number of tapfactors to be selected by the selectors SEL0-SEL15. The tap controllingunit 25 outputs any of TTRG(−3), TTRG(−2), TTRG(−1), TTRG(+1), TTRG(+2),and TTRG(+3) as the tap trigger signal TTRG. That is, in thisembodiment, the number of oversamples can be decremented or incrementedby “3” at maximum (maximum shift amount M of the number of oversamples,to be described later) during a period where the filter continues torespond to a single piece of input data DIN. In other respects, thebasic connections inside the oversampling FIR filter 22 are the same asthose of the first embodiment.

[0091] The tap factors selectable by the selectors SEL0-SEL15 areobtained from the following expressions (3) and (4). The expressions (3)and (4) signify the required number of tap factors prior and subsequentto the standard tap factors (in four), respectively.

OVR×(SL+1)−{(OVR−M)×SL+OVR}  (3)

OVR×(SL+1)−(OVR−M)×(SL+1)  (4)

[0092] (OVR: the number of oversamples,

[0093] M: maximum shift amount of the number of oversamples,

[0094] SL: the number of the selectors SEL0-SEL15)

[0095] Next, description will be given of the operation of theabove-described FIR filter. In the following example, selectors forselecting tap factors other than “0” will be exclusively shown fordescription.

[0096]FIG. 11 shows a case where the number of oversamples isdecremented by two from the standard and then incremented by three fromthe standard. Here, the tap trigger signal TTRG (−2) is first activatedin synchronization with the sampling trigger signal STRG, and the taptrigger signal TTRG (+3) is activated next.

[0097] According to on the information held in the flip-flops, the tapcontrolling unit 24 controls the selectors SEL0-SEL15 so as to selectsuccessive tap factors. Then, as shown by the thick frames in thediagram, the successive tap factors C1-C16 and the input data (DIN) arerespectively multiplied each other. As a result, the continuity inoutput response is maintained.

[0098]FIG. 12 shows a case where the number of oversamples isdecremented by two from the standard and then further decremented bythree from the standard. Here, the tap trigger signal TTRG (−2) is firstactivated in synchronization with the sampling trigger signal STRG, andthe tap trigger signal TTRG (−3) is activated next.

[0099]FIG. 13 shows a case where the number of oversamples isincremented by two from the standard and then further incremented bythree from the standard. Here, the tap trigger signal TTRG (+2) is firstactivated in synchronization with the sampling trigger signal STRG, andthe tap trigger signal TTRG (+3) is activated next.

[0100] In FIGS. 12 and 13, the successive tap factors C1-C16 and theinput data (DIN) are also multiplied each other as shown by the thickframes, so that the continuity of output responses is maintained.

[0101] This embodiment can offer the same effects as those obtained fromthe first embodiment described above. Besides, in this embodiment, themultiple decrement/increment in the number of oversamples can beswitched successively during a period where the filter continues torespond to a single piece of input data DIN.

[0102] Here, the number of selectable tap factors obtained by theabove-mentioned expressions (3) and (4) are required for the maximumchanges in the continuity limit of the filter responses.

[0103] The first embodiment described above has dealt with the casewhere the selectors SEL0-SEL4 receive tap factors from exterior.However, the present invention is not limited to such an embodiment. Forexample, circuits for prestoring tap factors may be formed inside theselectors SEL0-SEL4.

[0104] The above-described embodiments have dealt with the cases wherethe generator shown in FIG. 3 is formed in the transmitting circuit 6.However, the present invention is not limited to such an embodiment. Forexample, the generator may be formed aside from the semiconductorintegrated circuit implementing the FIR filter 2. Moreover, thegenerator 16 shown in FIG. 3 may be constituted by software.

[0105] The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and thescope of the invention. Any improvement may be made in part or all ofthe components.

What is claimed is:
 1. An oversampling FIR filter for filtering with aclock having a frequency higher than a frequency of accepting inputdata, comprising: a shift register having a plurality of holding partsconnected in cascade for sequentially accepting input data; a pluralityof selectors respectively formed corresponding to said holding parts forselecting, from a plurality of tap factors, a predetermined number oftap factors in synchronization with said clock; a plurality ofmultipliers formed respectively corresponding to said holding parts forrespectively multiplying said input data held in said holding parts, bysaid tap factors selected by said selectors corresponding to saidholding parts; and an adder for adding the multiplication results fromsaid multipliers and outputting the resultants as output data, andwherein said selectors change said predetermined number of tap factorsto be selected, in accordance with a change in the number ofoversamples, which is the number of tap factors to be multiplied by saidsingle input data.
 2. The oversampling FIR filter according to claim 1,wherein a part of said plurality of tap factors respectively selectableby said selectors adjacent to one another are shared by said selectors.3. The oversampling FIR filter according to claim 2, comprising a tapcontrolling unit for instructing said selectors said tap factor to beselected first in accordance with a change in said number ofoversamples.
 4. The oversampling FIR filter according to claim 2,wherein when said number of oversamples is changed, said tap controllingunit changes said tap factors selected by said selectors back to saidpredetermined tap factors used prior to the changing of said number ofoversamples, in which every time said input data is accepted, thechanges of said tap factors are performed in sequence, starting fromsaid selector corresponding to said holding part at the input side.
 5. Amethod for controlling an oversampling FIR filter for filtering with aclock having a frequency higher than a frequency of accepting inputdata, comprising the steps of: sequentially accepting input data at ashift register having a plurality of holding parts connected in cascade;sequentially selecting, from a plurality of tap factors, a predeterminednumber of tap factors in synchronization with said clock by a pluralityof selectors respectively formed corresponding to said holding parts;respectively multiplying said input data held in said holding parts bysaid tap factors selected by said selectors corresponding to saidholding parts; and adding the multiplication results and outputting theresultants as output data; and changing said predetermined number of tapfactors to be selected by said selectors, in accordance with a change inthe number of oversamples, which is the number of tap factors to bemultiplied by said single input data.
 6. The method for controlling anoversampling FIR filter according to claim 5, wherein a part of saidplurality of tap factors respectively selectable by said selectorsadjacent to one another are shared by said selectors.
 7. The method forcontrolling an oversampling FIR filter according to claim 6, comprisingthe step of instructing said selectors said tap factor to be selectedfirst in accordance with a change in said number of oversamples.
 8. Themethod for controlling an oversampling FIR filter according to claim 6,comprising the step of changing said tap factors selected by saidselectors back to said predetermined tap factors used prior to thechanging of said number of oversamples when said number of oversamplesis changed, in which every time said input data is accepted, saidchanges of said tap factors are performed in sequence, starting fromsaid selector corresponding to said holding part at the input side.
 9. Asemiconductor integrated circuit having an oversampling FIR filter,wherein said oversampling filter comprises: a shift register having aplurality of holding parts connected in cascade for sequentiallyaccepting input data; a plurality of selectors respectively formedcorresponding to said holding parts for selecting predetermined numberof tap factors in synchronization with said clock from a plurality oftap factors; a plurality of multipliers formed respectivelycorresponding to said holding parts for respectively multiplying saidinput data held in said holding parts by said tap factors selected bysaid selectors corresponding to said holding parts; and an adder foradding the multiplication results from said multipliers and outputtingthe resultants as output data, and wherein said selectors change saidpredetermined number of tap factors to be selected, in accordance with achange in the number of oversamples, which is the number of tap factorsto be multiplied by said single input data.
 10. A communication systemwherein data filtered with an oversampling FIR filter is transmitted,wherein said oversampling FIR filter comprises: a shift register havinga plurality of holding parts connected in cascade for sequentiallyaccepting input data; a plurality of selectors respectively formedcorresponding to said holding parts for selecting predetermined numberof tap factors in synchronization with said clock from a plurality oftap factors; a plurality of multipliers formed respectivelycorresponding to said holding parts for respectively multiplying saidinput data held in said holding parts by said tap factors selected bysaid selectors corresponding to said holding parts; and an adder foradding the multiplication results from said multipliers and outputtingthe resultants as output data, and wherein said selectors change saidpredetermined number of tap factors to be selected, in accordance with achange in the number of oversamples, which is the number of tap factorsto be multiplied by said single input data.